Recent developments of the 3GPP Long Term Evolution (LTE) facilitate accessing local IP based services in the home, office, public hot spot or even outdoor environments. One of the important use cases for the local IP access and local connectivity involves the direct communication between devices in the close proximity (typically less than a few 10 s of meters, but sometimes up to a few hundred meters) of each other.
This direct communication may be called D2D communication or direct mode or ProSe, communication (Proximity Service) and may enable a number of potential gains over the traditional cellular technique, because ProSe communication may involve communication over a much shorter distance between involved devices than cellular communication between devices communicating via a cellular access point (e.g., base station or eNB):                Capacity gain: First, radio resources (e.g. OFDM resource blocks) between the ProSe and cellular layers may be reused (reuse gain). Second, a ProSe link uses a single hop between the transmitter and receiver points as opposed to the 2-hop link via a cellular AP (hop gain).        Peak rate gain: due to the proximity and potentially favorable propagation conditions high peak rates could be achieved (proximity gain);        Communication Latency gain: When the UEs communicate over a direct link, eNB forwarding is short cut and the end-to-end latency can decrease.        
ProSe transmissions of such direct communication may use the same frame structure as the frame structure that is defined for uplink and downlink when UEs are in network coverage (in cellular communication); however, the ProSe transmissions may be restricted to a sub-set of the uplink resources in time and frequency domain. For LTE TDD system, the ProSe transmissions may be used in uplink sub-frames (as determined/used/scheduled in the context of cellular communication, e.g. by the network or network node).
It should be noted that in the following, the term ProSe (Proximity Services) communication or related terms may be used as examples for direct or D2D (Device-to-Device) communication in the context of LTE. However, this is by way of example only and generally, D2D communication or Direct Mode communication or related terms may be used instead unless specifically stated otherwise.